1. Field of the Invention
The present invention relates to an echo canceller for multi-channel processing which cancels echoes contained in transmission signals by subtracting echo replicas which are convolutions of estimated impulse responses of echo paths with receive signals.
2. Description of the Related Art
For example, a conventional multi-channel echo canceller having a plurality of single-channel echo cancellers which is disclosed in Japanese Patent Unexamined Publication No. Hei 4-331512, as shown in FIG. 30, includes receive signal input/output terminals 11 to 1n/21 to 2n that input receive signals y1(i) to yn(i) from far-end sides at each sampling time and outputs them to near-end sides. Transmission signal input/output terminals 31 to 3n/41 to 4n input from the near-end path transmission signals x1(i) to xn(i) where near-end talking signals are superimposed on the echo signals that are reflections of the receive signals y1(i) to yn(i) from the echo paths through the transmission signal input terminals 31 to 3n, and output residual signals e1(i) to en(i) which are subtractions of the echo replicas r1(i) to rn(i) from the transmission signals x1(i) to xn(i).
Finite impulse response (FIR) filter sections 51b to 5nb input y1(i) to yn(i) from the receive-signal input terminals 11 to 1n and store filter coefficients which are estimated impulse responses of the echo paths and produce the echo replicas r1(i) to rn(i) by calculating convolutions of the filter coefficients and y1(i) to yn(i), and output e1(i) to en(i). Subtractors 61 to 6n input x1(i) to xn(i) from the transmission-signal input terminals 3i to 3n and r1(i) to rn(i) from the FIR filter sections 51b to 5nb and produce the residual signals e1(i) to en(i) by subtracting r1(i) to rn(i) from x1(i) to xn(i), respectively, and output e1(i) to en(i).
Double-talk detection circuits 81 to 8n monitor the powers of y1(i) to yn(i), x1(i) to xn(i) and r1(i) to rn(i), and judge the presence/absence of the double-talk state (the powers of both the transmission signal and the receive signal are high)/the silent state (the power of the receive signal is low), and output detection signals of "0" in either the double-talk state or the silent state, and output signals of "1" in the other states to filter coefficients update circuits 71 to 7n.
Filter coefficient update circuits 71 to 7n conduct update processing in accordance with detection signals from the double-talk detection circuits 81 to 8n in such a manner that the filter coefficients (impulse response estimated values) stored in the FIR filter sections 51b to 5nb are controlled so that e1(i) to en(i) approach 0 in no double-talk state and in no silent state, and so that the update processing is stopped in the double-talk state or in the silent state.
The above-mentioned conventional multi-channel echo canceller is of the system (each-channel-independent echo canceling system) for canceling echoes by a processing complexity n times as much as the processing complexity for one channel.
The operation of the FIR filter section 5zb (z=1 to n) is, for example, as shown in FIG. 31 of Japanese Patent Unexamined Publication No. Hei 2-305231. The receive-signal register 51z stores yz(i) newly inputted from the receive-signal input terminal 1z after cutting off the oldest yz(i-h) (h is a natural number and the number of stored samples) and hold finite predetermined h number of samples of yz(i), yz(i-1), . . . yz(i-h+1). The filter coefficient register 52z stores the filter coefficient for the number h of samples which is the same as that of the receive-signal register 51z. In this example, when the echo signal contained in the transmission signal xz(i) is completely removed by the echo replica rz(i) whereby the residual signal ez(i) becomes 0, the complete estimation of the echo path characteristic can be performed, and the filter coefficient ax.sub.k (i) becomes a value identical with that of ax.sub.k (i-1) before one sample time and held as it is. Furthermore, a convolution calculator 53z calculates convolution of receive signals yz (i-k) with the filter coefficients az.sub.k (i) to product an echo replica rz(i)=.SIGMA..sub.k az.sub.k (i) yz(i-k) (k=0 to h-1).
Also, in the case of two-channel echo canceller, conventional multi-channel echo canceller is disclosed in Japanese Patent Unexamined Publication No. Hei 4-284732, as shown in FIG. 32. Receive-signal input terminals 801/802 and receive-signal output terminals (speakers) 803/804 input the receive signals y1(i)/y2(i) from the far-end side at each sampling time i and output them to the near-end side. Transmission-signal input terminals (microphones) 805/806 and transmission-signal output terminals 807/808 input transmission signals x1(i)/x2(i) containing the echo signals that are reflections of the receive signals y1(i) to y2(i) from the echo path extending from the speakers 803/804 to the microphones 805/806 from the near-end side, and output residual signals e1(i)/e2(i) which are subtractions of the echo replicas r1(i) to rn(i) from the transmission signals x1(i)/x2(i). Adaptive filters 809/810 produce echo replicas r1(i)/r2(i) which are controlled so that e1(i)/e2(i) approach 0. A phrase-difference detector 814 estimates a phase difference between y1(i) to y2(i). A selector 813 selects y1(i) or y2(i) whose phase is faster than that of another and outputs it to adaptive filters 809/810.
The above-mentioned conventional multi-channel echo canceller is of the system (each-channel dependent echo canceling system) in which a plurality of echo paths are estimated through adaptive filter operation not independent in each channel to cancel the echo.
Also, in the case of a multi-point conference echo canceller, a conventional multi-channel echo canceller is disclosed in Japanese Patent Unexamined Publication No. Hei 2-87861, as shown in FIG. 33. Receive-signal input terminals 901 to 90n and receive-signal output terminals (speakers) 911 to 91n input receive-signals y1(i) to yn(i) from the far-end side at each sampling time i and output them to the near-end side through loss insertion circuits 961 to 96n. Transmission-signal input/output terminals 920/921 input from the near-end side a transmission signal x(i) containing the echo signals that are reflections of the receive signals y1(i) to yn(i) from the echo path extending from the speakers 911 to 91n to the transmission input terminal 920, and output the residual signal e(i) that is a subtraction of an echo replica r1(i) from a transmission signal x1(i) to the far-end side. A receive-talk channel selection circuit 942 select m (m is an integral number smaller than n) channels in which the levels of the receive signals are larger than those in other channels. Adaptive filters 931 to 93m produce the echo replicas r1(i) to rm(i) calculating convolutions of the receive signals and the filter coefficients stored in storage circuits 951 to 95n in selected m channels, and update the filter coefficients so that the residual signals e(i) approaches 0.
The above-mentioned multi-channel echo canceller is of the system (each-channel limited echo canceling system) in which adaptive filter operation is limited to the m-channels among all the n-channels to cancel the echo.
In the above-mentioned conventional multi-channel echo cancellers as shown in FIGS. 30 and 32, the amount of arithmetic operation is considerably increased in proportion to the number of channels because the adaptive filters in all channels always operate. Furthermore, in the above-mentioned conventional multi-channel echo cancellers as shown in FIG. 33, there arises such a problem that the adaptive filter may not permanently operate in a specified channel where the receive signal continues to be in a low level.